Bacteria containing lambda N minus prophages die when lambda is derepressed, while lysogens with a large number (6 or more) survive but undergo a change in morphology called conversion. Derepression causes an increase in the number of lambda genotypes in all lambda N minus lysogens. We will attempt to obtain physical and biochemical evidence for the endoreplication of lambda prophages and of host genes adjacent to lambda N minus prophages. By using mutations and deletions of host genes near the lambda attachment site, we will determine whether the phenotypic changes after derepression are dependent on these genes. We will study the behavior of lysogens with lambda N minus prophages inserted at other sites on the bacterial chromosome. Lambda prophages with mutations and deletions of "early genes" will be tested for their effect on the host after derepression. In cultures of converted lambda N minus polylysogens, there appears to be a constant turnover of prophages. Using genetically marked prophages and superinfecting phage, we will study the mechanisms responsible for maintaining a large number of prophages in the derepressed state. Lysogens of certain lambda cIts mutants appear to undergo partial derepression after heating. The expression of genes of the trp operon that have been put under the control of temperature-sensitive lambda repressors will be studied. The affinity for lambda DNA of various Cits repressors will be determined in vitro.